An electric machine such as an electrical motor, power generation system, genset, or the like, is generally used to convert one form of energy into another and may operate in a motoring mode to convert electrical input into rotational or otherwise mechanical output, or operate in a generating mode to convert rotational or otherwise mechanical input into electrical output. Among the various types of machines available for use with an electric drive, switched reluctance (SR) machines have received great interest for being robust and cost-effective. While currently existing systems and methods for controlling such electric machines provide adequate control, there is still room for improvement.
Among other factors, proper determination of the position of the rotor relative to the stator of the SR machine, while at rest or at an otherwise substantially low machine speed, is important to the performance and efficiency of the SR machine. Some conventional control schemes rely on a mechanically aligned speed wheel and sensors to detect and determine the position of the rotor relative to the stator at machine standstill or low speed operations. These control schemes typically require costly and complex implementations and are still susceptible to error. For instance, an error of 2 degrees in the detected mechanical rotor position of an SR machine, caused by a skewed sensor, a mechanical misalignment of the speed wheel, or the like, may correspond to a 0.5% decrease in efficiency of the electric drive assembly at full load.
Sensorless control schemes can also be used to derive the rotor position using electrical characteristics of the SR machine. For example, the control system of U.S. Pat. No. 5,525,886 to Lyons, et al. injects a current signal having fixed voltage frequency and varying current height to compute a total voltage flux in the SR machine. Lyons then determines the rotor position based on the voltage flux and the phase current. While Lyons may provide more simplicity over sensor-based schemes, the voltage integrator in Lyons still accumulates offset errors at least during the measurement path and in estimating the voltage flux. Such error accumulation can be compounded and adversely affects the accuracy of rotor position detection especially during low machine speeds and standstill.
Accordingly, there is a need to provide a control system or scheme for controlling SR machines that is less costly and easier to implement without compromising overall performance. Moreover, there is a need to provide a control system or scheme that does not rely on rotor position sensors, and further, substantially reduces accumulation of offset errors to provide for more accurate, more reliable and more efficient operation of an SR machine at standstill or low machine speeds. The systems and methods disclosed are directed at addressing one or more of these needs.